The present invention relates to a laser diode mount having a thermal coupling for a laser diode heat sink using a shape memory alloy to obtain a variable thermal impedance by the regulation of the air gap thickness between the laser diodes and the heat sink.
Laser diode arrays provide greatly increased optical pumping efficiency and life compared with flash lamp pumping of lasers. However, the laser diode arrays provide a maximum pumping efficiency over a relatively narrow temperature band. This results in the need to heat up and then maintain the diodes within their operational temperature bandwidth. When the diodes are at their normal operational temperature, heat must be continuously dissipated into a forced air or forced liquid heat sink. In many laser systems, rapid operational capability of the laser is required and in addition many portable systems utilize batteries so that large amounts of power are not available to provide for the diode heating. If the complete diode head system and heat sinks are heated up, a large amount of power will be required because of the high thermal capacity of the heat and heat sink. Thus, a variable thermal impedance device which isolates the diodes from the head and heat sink during warm-up and then connects the diodes to the heat sink once an operational temperature is reached is a desirable goal.
In the past, various actuation methods have been available to provide a variable thermal impedance between diodes and a heat sink. These may be divided into two categories, active and passive. The active devices provide a variable thermal impedance using mechanisms driven by motors or solenoids and require extra power consumption in addition to adding weight, size and complexity to the system. Passive devices include the use of wax or fluid thermal expansion and bimetallic actuators. Wax and fluid filled actuators tend to have large thermal capacity and slow response times. Bimetallic devices can provide relatively large motions but, at the same time, provide relatively low force.
The present invention is for a variable thermal impedance device used in connection with a diode head system and heat sink which utilizes shape memory alloys which are alloys that undergo a reversible martensite-austenite crystal phase transformation below a certain transformation temperature which determined by the alloy composition. The shape memory alloy is martensitic, weak, and very easily deformed to enlarge strain level without permanent damage. When the material is heated to an austenite phase above the transformation temperature, deformation goes away and the material returns to the original shape. The austenite-phase material can be up to ten times stronger than its martensitic counterpart. A shape memory alloy, such as a nickel titanium alloy, can be used for this purpose.
Prior U.S. patents which show various ways of maintaining a temperature bandwidth or cooling laser diode arrays can be seen in U.S. Pat. No. 4,792,957 to Kollanyi for a laser temperature controller which uses a bridge circuit connected with a thermistor device in an active temperature controller. In the U.S. Pat. No. 4,550,333to Ridder et al., a light emitting semiconductor mount includes a metal heat sink support having a top surface positioned into three steps with the semiconductor mounted to the central step and a dielectric block mounted to the lower step. An electrical coupling means couples the semiconductor to the lower step. In the Yoshikawa U.S. Pat. No. 4,571,728, a temperature controlled device for a semiconductor laser has a temperature sensitive resistor for detecting the temperature of a semiconductor laser and a Peltier element connected to the semiconductor laser and controlled with regard to the direction of the heat transmission in accordance with the resistance value of the temperature sensitive resistor. In the U.S. Pat. No. 4,856,015 to Matsui, et al., a semiconductor laser array device that allows temperature distribution on the light emitting area is shown. In the Watanabe U.S. Pat. No. 4,689,659, a temperature controller for a semiconductor device is electronically controlled and is responsive to an output signal from a temperature sensor to control the temperature of the semiconductor device so as to be substantially constant and uses a temperature sensor to detect the temperature of a semiconductor device for producing the signal while controlling the temperature. In the Yonezu et al. U.S. Pat. No. 3,946,334, an injection semiconductor laser device includes an ohmic metal film on the side of the heat sink near the active region and is plated with a layer of thermally conducted material to protect the laser from being damaged when the laser device is bonded to the heat sink. The Fisher et al. U.S. Pat. No. 5,029,335, is for a heat dissipating device for laser diodes for dissipating waste heat produced by the solid state device and includes a base member in thermal contact with a solid state device and a plurality of elongated heat conducting elements extending outwardly from the base member. In the Martin U.S. Pat. No. 4,901,324, a heat transfer device for cooling and transferring heat from a laser diode device and associated heat generating elements is provided and includes an elongated closed ended heat pipe having one portion in a closely spaced proximity and in heat conductive relationship to the laser medium and a second remote end portion in a heat conducting relationship to a heat sink. A heat transfer fluid can move through the heat pipe to remote locations for conduction of the heat therein into the heat sink.
The present invention is for a passive thermal actuator using a shape memory alloy to obtain a variable thermal impedance by the regulation of an air gap thickness between a laser diode array and a heat sink. The device permits rapid warm-up of the diodes by providing thermal isolations from the heat sink during the warm-up and then control of the heat transfer to the heat sink when the diodes reach their normal operating temperature. The power required to bring the diodes up to operating temperatures is thus greatly reduced, resulting in a lowering of a system power requirements and weight.